Vitamin D is a fat-soluble vitamin, the two major forms of which are vitamin D2 and vitamin D3. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation. Together with calcium, vitamin D also helps protect older adults from osteoporosis.
Vitamin D is not naturally present in many foods, therefore it is added to some foods, and is available as a dietary supplement, often combined with calcium, as the latter cannot be absorbed without this vitamin.
Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active
Vitamin D is essential for promoting calcium absorption in the gut and maintaining adequate calcium and phosphate concentrations to enable normal bone mineralization, repair broken bones, and prevent osteoporosis.
Vitamin D has other roles in human health, including modulation of neuromuscular and immune function and reduction of inflammation. Recent research seems to indicate that Vitamin D could play a role in the prevention of colon, prostate and breast cancers and possibly other forms of the disease. However, this research is still in the exploratory stage.
As people age, skin cannot synthesize vitamin D as efficiently and the kidney is less able to convert vitamin D to its active hormone form. Recommended amounts for adults is 5 micrograms (mcg), but after 50 the recommended amount doubles to 10 mcg and after 70 rises to 15 mcg.
Selected Food Sources of Vitamin D
IUs per serving*
Cod liver oil, 1 tablespoon
Salmon, cooked, 3.5 ounces
Mackerel, cooked, 3.5 ounces
Tuna fish, canned in oil, 3 ounces
Sardines, canned in oil, drained, 1.75 ounces
Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup
Margarine, fortified, 1 tablespoon
Ready-to-eat cereal, fortified with 10% of the DV for vitamin D, 0.75-1 cup (more heavily fortified cereals might provide more of the DV)
Egg, 1 whole (vitamin D is found in yolk)
Liver, beef, cooked, 3.5 ounces
Cheese, Swiss, 1 ounce
*IUs = International Units. **DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration to help consumers compare the nutrient contents of products within the context of a total diet. The DV for vitamin D is 400 IU for adults and children age 4 and older. Food labels, however, are not required to list vitamin D content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
The U.S. Department of Agriculture's Nutrient Database Web site: (http://www.nal.usda.gov/fnic/foodcomp/search/) lists the nutrient content of many foods; relatively few have been analyzed for vitamin D content.
Most people meet their vitamin D needs through exposure to sunlight. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis Sunscreens with a high sun protection factor appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly. Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.
The amount of exposure for adequate vitamin D synthesis recommended by recent research is approximately 5-30 minutes of non-protected sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back, or the moderate use of tanning beds. Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.
Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight. UV radiation is a carcinogen responsible for most of the skin cancers and lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes. It is not known whether a desirable level of regular sun exposure exists that imposes no (or minimal) risk of skin cancer over time. The American Academy of Dermatology and other authorities worldwide, advise that photo protective measures be taken, including the use of sunscreen, whenever one is exposed to the sun.